A bulk acoustic resonator with vertical electrodes for wideband filters

Radiofrequency front ends for current and next generation (5G and 6G) wireless communication demand acoustic filters that combine wide bandwidth, high power capability, and thermal stability. Existing surface and bulk acoustic wave technologies face inherent trade-offs between electromechanical coupling, lithographic tunability, and robustness. Here, we introduce the bulk acoustic resonator with vertical electrodes (VBAR), a device that combines the advantages of suspended and solidly mounted resonators. VBARs use lithium niobate ridges with sidewall electrodes to excite a shear-horizontal bulk acoustic resonance, providing frequency control through lithography in a configuration that is mechanically anchored to the substrate. Fabricated VBARs exhibit electromechanical coupling coefficients exceeding 30% in the 2-4 GHz range, enabling ladder filters with fractional bandwidths of nearly 20%. While further optimization is necessary to minimize losses, the VBAR concept offers an alternative route toward wideband and robust radiofrequency filters for next-generation wireless systems.